There's probably more people using WhatsApp securely in authoritarian hellholes and nightmarish war zones around the world than many of the alternatives you are thinking of combine.
Personally I wouldn't use any chat service for super sensitive conversations about company comms and IP that didn't have strong encryption as well as limitations on how messages can be backed up as well as disappearing messages. I think Signal is the only system that is semi-popular that does this where by default backups don't leave the device unencrypted.
I wouldn't be surprised if Meta was the number one way people have private conversations on planet earth.